# add_custom_command

Add a custom build rule to the generated build system.

There are two main signatures for `add_custom_command`.

## Generating Files

The first signature is for adding a custom command to produce an output:

    add_custom_command(OUTPUT output1 [output2 ...]
                       COMMAND command1 [ARGS] [args1...]
                       [COMMAND command2 [ARGS] [args2...] ...]
                       [MAIN_DEPENDENCY depend]
                       [DEPENDS [depends...]]
                       [BYPRODUCTS [files...]]
                       [IMPLICIT_DEPENDS <lang1> depend1
                                        [<lang2> depend2] ...]
                       [WORKING_DIRECTORY dir]
                       [COMMENT comment]
                       [DEPFILE depfile]
                       [VERBATIM] [APPEND] [USES_TERMINAL]
                       [COMMAND_EXPAND_LISTS])

This defines a command to generate specified `OUTPUT` file(s). A target created in the same directory (`CMakeLists.txt` file) that specifies any output of the custom command as a source file is given a rule to generate the file using the command at build time. Do not list the output in more than one independent target that may build in parallel or the two instances of the rule may conflict (instead use the `add_custom_target()` command to drive the command and make the other targets depend on that one). In makefile terms this creates a new target in the following form:

    OUTPUT: MAIN_DEPENDENCY DEPENDS
            COMMAND

The options are:

`APPEND`
  Append the `COMMAND` and `DEPENDS` option values to the custom command for the first output specified. There must have already been a previous call to this command with the same output. The `COMMENT`, `MAIN_DEPENDENCY`, and `WORKING_DIRECTORY` options are currently ignored when APPEND is given, but may be used in the future.
`BYPRODUCTS`

  Specify the files the command is expected to produce but whose modification time may or may not be newer than the dependencies. If a byproduct name is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory. Each byproduct file will be marked with the `GENERATED` source file property automatically.

  Explicit specification of byproducts is supported by the `Ninja` generator to tell the `ninja` build tool how to regenerate byproducts when they are missing. It is also useful when other build rules (e.g. custom commands) depend on the byproducts. Ninja requires a build rule for any generated file on which another rule depends even if there are order-only dependencies to ensure the byproducts will be available before their dependents build.

  The `BYPRODUCTS` option is ignored on non-Ninja generators except to mark byproducts `GENERATED`.

`COMMAND`

  Specify the command-line(s) to execute at build time. If more than one `COMMAND` is specified they will be executed in order, but not necessarily composed into a stateful shell or batch script. (To run a full script, use the `configure_file()` command or the `file(GENERATE)` command to create it, and then specify a `COMMAND` to launch it.) The optional `ARGS` argument is for backward compatibility and will be ignored.

  If `COMMAND` specifies an executable target name (created by the `add_executable()` command) it will automatically be replaced by the location of the executable created at build time. If set, the `CROSSCOMPILING_EMULATOR` executable target property will also be prepended to the command to allow the executable to run on the host. (Use the `TARGET_FILE` `generator expression` to reference an executable later in the command line.) Additionally a target-level dependency will be added so that the executable target will be built before any target using this custom command. However this does NOT add a file-level dependency that would cause the custom command to re-run whenever the executable is recompiled.

  Arguments to `COMMAND` may use `generator expressions`. References to target names in generator expressions imply target-level dependencies, but NOT file-level dependencies. List target names with the `DEPENDS` option to add file-level dependencies.

`COMMENT`
  Display the given message before the commands are executed at build time.
`DEPENDS`

  Specify files on which the command depends. If any dependency is an `OUTPUT` of another custom command in the same directory (`CMakeLists.txt` file) CMake automatically brings the other custom command into the target in which this command is built. If `DEPENDS` is not specified the command will run whenever the `OUTPUT` is missing; if the command does not actually create the `OUTPUT` then the rule will always run. If `DEPENDS` specifies any target (created by the `add_custom_target()`, `add_executable()`, or `add_library()` command) a target-level dependency is created to make sure the target is built before any target using this custom command. Additionally, if the target is an executable or library a file-level dependency is created to cause the custom command to re-run whenever the target is recompiled.

  Arguments to `DEPENDS` may use `generator expressions`.

`COMMAND_EXPAND_LISTS`
  Lists in `COMMAND` arguments will be expanded, including those created with `generator expressions`, allowing `COMMAND` arguments such as `${CC} "-I$<JOIN:$<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>,;-I>" foo.cc` to be properly expanded.
`IMPLICIT_DEPENDS`
  Request scanning of implicit dependencies of an input file. The language given specifies the programming language whose corresponding dependency scanner should be used. Currently only `C` and `CXX` language scanners are supported. The language has to be specified for every file in the `IMPLICIT_DEPENDS` list. Dependencies discovered from the scanning are added to those of the custom command at build time. Note that the `IMPLICIT_DEPENDS` option is currently supported only for Makefile generators and will be ignored by other generators.
`MAIN_DEPENDENCY`
  Specify the primary input source file to the command. This is treated just like any value given to the `DEPENDS` option but also suggests to Visual Studio generators where to hang the custom command. Each source file may have at most one command specifying it as its main dependency. A compile command (i.e. for a library or an executable) counts as an implicit main dependency which gets silently overwritten by a custom command specification.
`OUTPUT`
  Specify the output files the command is expected to produce. If an output name is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory. Each output file will be marked with the `GENERATED` source file property automatically. If the output of the custom command is not actually created as a file on disk it should be marked with the `SYMBOLIC` source file property.
`USES_TERMINAL`
  The command will be given direct access to the terminal if possible. With the `Ninja` generator, this places the command in the `console` `pool`.
`VERBATIM`
  All arguments to the commands will be escaped properly for the build tool so that the invoked command receives each argument unchanged. Note that one level of escapes is still used by the CMake language processor before add_custom_command even sees the arguments. Use of `VERBATIM` is recommended as it enables correct behavior. When `VERBATIM` is not given the behavior is platform specific because there is no protection of tool-specific special characters.
`WORKING_DIRECTORY`

  Execute the command with the given current working directory. If it is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory.

  Arguments to `WORKING_DIRECTORY` may use `generator expressions`.

`DEPFILE`
  Specify a `.d` depfile for the `Ninja` generator. A `.d` file holds dependencies usually emitted by the custom command itself. Using `DEPFILE` with other generators than Ninja is an error.

## Build Events

The second signature adds a custom command to a target such as a library or executable. This is useful for performing an operation before or after building the target. The command becomes part of the target and will only execute when the target itself is built. If the target is already built, the command will not execute.

    add_custom_command(TARGET <target>
                       PRE_BUILD | PRE_LINK | POST_BUILD
                       COMMAND command1 [ARGS] [args1...]
                       [COMMAND command2 [ARGS] [args2...] ...]
                       [BYPRODUCTS [files...]]
                       [WORKING_DIRECTORY dir]
                       [COMMENT comment]
                       [VERBATIM] [USES_TERMINAL])

This defines a new command that will be associated with building the specified `<target>`. The `<target>` must be defined in the current directory; targets defined in other directories may not be specified.

When the command will happen is determined by which of the following is specified:

`PRE_BUILD`
  On Visual Studio Generators, run before any other rules are executed within the target. On other generators, run just before `PRE_LINK` commands.
`PRE_LINK`
  Run after sources have been compiled but before linking the binary or running the librarian or archiver tool of a static library. This is not defined for targets created by the `add_custom_target()` command.
`POST_BUILD`
  Run after all other rules within the target have been executed.

Note

Because generator expressions can be used in custom commands, it is possible to define `COMMAND` lines or whole custom commands which evaluate to empty strings for certain configurations. For Visual Studio 2010 (and newer) generators these command lines or custom commands will be omitted for the specific configuration and no “empty-string-command” will be added.

This allows to add individual build events for every configuration.